(a) Field of the Invention
The present invention relates to a demodulating device and method in an orthogonal frequency division multiple access (OFDMA) communication system. More particularly, the present invention relates to a demodulating device and method for demodulating a plurality of data bursts from one frame.
(b) Description of the Related Art
In recent years, a wideband wireless access system that supports the mobility of a subscriber's terminal in addition to wireless data communication based on a fixed access point, such as a LAN, has been developed OFDMA has been adopted as the communication mode for a physical layer in the IEEE 802.16 standard among the wideband wireless access systems being currently developed.
Strictly, OFDMA means an OFDM-FDMA communication system, in which sub-carriers having a plurality of orthogonal frequencies are multiplexed by using a plurality of sub-channels. The wideband wireless access system differs from the OFDM-TDMA communication system that transmits data to user terminals through each time slot in that the same modulation level and channel scheme are transmitted as one burst. In the following description, the OFDM-FDMA system is simply referred to as an OFDMA system.
FIG. 1 is a diagram illustrating an example of a frame used in the OFDMA communication system.
FIG. 1, the horizontal axis is a time axis and is divided in the unit of symbols, and the vertical axis is a frequency axis and is divided in the unit of sub-channels. Each sub-channel is a set of a plurality of sub-carriers. Specifically, in an OFDMA physical layer, active carriers are classified into a plurality of groups, and the groups of active carriers are transmitted to different receivers. A group of sub-carriers transmitted to one receiver is called a sub-channel. The carriers forming each sub-channel may be adjacent to each other or separated at equal intervals from each other.
Referring to FIG. 1, a preamble symbol is positioned at the head of each frame, and is used to acquire time synchronization and frequency synchronization, to search a cell including a terminal, and to estimate a channel.
MAP information follows the preamble symbol. The MAP information includes various information items, such as information required for demodulation and information on the state of a base station. That is, the MAP information includes information on the position and size of a data burst allocated to the terminal and information on a modulation mode. Since the MAP information needs to be demodulated such that the user gets the MAP information, the MAP information is transmitted through all of the sub-channels of the data symbol.
Several user data bursts follow the MAP information. User data is composed of several data bursts according to users and purposes, and is two-dimensionally allocated in the OFDMA system. The data burst has a sub-channel composed of a plurality of sub-carriers as a basic unit.
The data bursts are transmitted by using different modulation and coding schemes. For example, in FIG. 1, burst Nos. 1 and 2 are modulated by QPSK in order to transmit broadcasting information to all users in a cell, and then transmitted using 1/12 channel coding. Burst No. 3 is modulated by 64 QAM and then transmitted to user terminals whose channel conditions are good by using ⅚ channel coding.
When a terminal receives one data burst or one broadcasting information item, a demodulating device has a simple structure. However, when a plurality of data bursts included in one frame are simultaneously received, the structure of the demodulating device of the terminal is complicated. The demodulating device of the terminal should demodulate several data bursts when data for various purposes is simultaneously transmitted to one user at various transmission speeds (for example, 28.8 kbps and 1.44 Mbps). In addition, the demodulating device of the terminal should demodulate several data bursts in an OFDM system that reads several carriers and reconfigures one information item.
FIG. 2 is a block diagram illustrating the structure of a demodulating device of a terminal that simultaneously receives a plurality of data bursts included in one frame.
As shown in FIG. 2, the demodulating device of the terminal includes an A/D converter 10, a fast Fourier transformer (FFT) 11, a reorder buffer 12, a demodulator 13, a slot buffer 14, and a channel decoder 15. The demodulator 13 includes an equalizer and a QAM demapper.
When an OFDMA frame is received, the A/D converter 10 of the demodulating device shown in FIG. 2 converts the received signal into a digital signal, and the FFT 11 performs a fast Fourier transform on the digital signal Then, the transformed signal is stored in the reorder buffer 12, and the demodulator 13 performs channel estimation and equalization on the stored data. Subsequently, the data is subjected to QAM demapping and is then output. The data output from the demodulator 13 is stored in the slot buffer 14, and then decoded by the channel decoder 15. Then, the data is demodulated.
The number of A/D converters 10, fast Fourier transformers 11, and reorder buffers 12 is fixed since they are not concerned with the number of data bursts to be demodulated. However, the number of demodulators 13, slot buffers 14, and channel decoders 15 depends on the number of data bursts to be demodulated. That is, when N data bursts, which is a maximum number, are simultaneously demodulated from one frame N demodulators 13, N slot buffers 14, and N channel decoders 15 are needed.
FIG. 3 is a flowchart illustrating the operation of the demodulating device receiving the frame shown in FIG. 1 and demodulating four data bursts, that is, data burst No. 1 to data burst No. 4.
As shown in FIG. 3, a fast Fourier transform (FFT) is performed on the received data, and the transformed data is stored in the reorder buffer 12. Then, the demodulators 13 corresponding to the data bursts including the sub-channels sequentially perform channel estimation, equalization, and QAM demapping on the sub-channel data stored in the reorder buffer 12, and the processed data is stored in the slot buffer 14. That is, when sub-channel Nos. 1 and 2 corresponding to data burst No. 2 and sub-channel No. 3 corresponding to data burst No. 3 are simultaneously received, a fast Fourier transform is performed on the data corresponding to the sub-channel Nos. 1 to 3, and the transformed data is stored in the reorder buffer. Then, data corresponding to the sub-channel Nos. 1 and 2 is stored in the slot buffer 14 through the demodulator 13 corresponding to the data burst No. 2, and data corresponding to the sub-channel No. 3 is stored in the slot buffer 14 through the demodulator 13 corresponding to the data burst No, 3. When QAM demapping is completely performed on all the data of the data bursts, the channel decoder 15 performs channel decoding on the data stored in the slot buffer 14.
As described above, the demodulating device including N demodulators 13, N slot buffers 14, and N channel decoders 15 in order to demodulate N data bursts included in one data frame has a complicated hardware structure, which results in an increase in manufacturing costs.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.